The present invention relates to a heating device for heating a recording sheet being transported on a sheet feed path.
In heating a recording sheet with a heating roller, the temperature of the heater provided within the heating roller is established so that the surface temperature of the heating roller falls within a predetermined allowable range. When the heating roller is used in a fixing device for example, the temperature of the heater provided in the heating roller is established so that the surface temperature of the heating roller falls within a range between the upper limit and the lower limit of the fixing temperature.
However, the surface temperature of the heating roller is influenced by the frequency of contacts with recording sheets. A portion of the heating roller surface which frequently contacts recording sheets is subject to cooling due to transfer of heat to recording sheets. In contrast, a portion of the heating roller surface which does not contact recording sheets very frequently is not cooled readily. In heating small-sized recording sheets successively for example, axially opposite end portions of the heating roller do not contact the recording sheets and hence are not cooled readily. For this reason, the surface temperature of the opposite end portions of the heating roller sometimes becomes higher than necessary in heating such small-sized recording sheets successively even when the established temperature of the heater disposed within the heating roller is proper. In the case where the heating roller is used in the fixing device, such an inconvenience that a recording sheet or fused toner twines around the heating roller is likely to occur when the surface temperature of the heating roller becomes higher than the upper limit of fixing temperature.
In attempt to solve this problem, one prior-art heating device is designed to set the surface temperature of a heating roller using two heaters, as in the invention described in Japanese Laid-Open Patent Application No. 2001-175120.
FIGS. 1A and 1B show the heat distribution characteristics of heaters used in a conventional heating device and the surface temperature of a heating roller used in the conventional heating device. In FIG. 1A, the abscissa represents locations along the axis of the heating roller while the ordinate represents surface temperatures of the heating roller. “Max”, “Min” and “F” in FIG. 1A represent the upper limit, lower limit and established value, respectively, of fixing temperature. In FIG. 1B, the abscissa represents locations along the axis of the heating roller while the ordinate represents the heat distribution characteristic of each heater. The heating device of FIGS. 1A and 1B includes a main heater and a sub-heater. As plotted by line X1 in FIG. 1B, the main heater has such a heat distribution characteristic as to provide a high temperature in an axially central portion of the heating roller and a low temperature in axially opposite end portions of the heating roller. As plotted by line X2 in the same figure, the sub-heater has such a heat distribution characteristic as to provide a low temperature in the axially central portion of the heating roller and a high temperature in the axially opposite end portions of the heating roller.
Such an arrangement performs temperature control over the heating roller in accordance with recording sheet sizes. For example, only the main heater is actuated when heating is to be performed on a recording sheet having a width smaller than L2 axially of the heating roller. Such conventional temperature control can prevent the surface temperature of the axially opposite end portions of the heating roller from becoming higher than necessary even when small-sized recording sheets are heated successively.
In these years, however, frequent use is made of recording sheets of very small sizes (hereinafter will be referred to as “very small-sized recording sheet(s)”), such as postcards, which are much smaller than the aforementioned small-sized recording sheets, giving rise to a new problem. In FIG. 1A, “L1” represents the width of a very small-sized recording sheet which extends parallel with the axis of the heating roller.
In successive heating of very small-sized recording sheets, a portion of the surface of the heating roller which is heated by the main heater but does not contact the very small-sized recording sheets is heated to a temperature higher than necessary even under the aforementioned temperature control, as the case may be.
In FIG. 1A, curve T1 plots the surface temperature of the heating roller before heating of recording sheets and curve T2 plots the surface temperature of the heating roller after successive heating of very small-sized recording sheets. As can be seen from FIG. 1A, the surface temperature of a portion of the heating roller becomes higher than the upper limit of fixing temperature after successive heating of the very small-sized recording sheets.
According to the prior art, an additional heater dedicated to heating of very small-sized recording sheets need be provided within the heating roller in order to maintain the surface temperature of the heating roller within a proper range even in successive heating of very small-sized recording sheets. This results in an inconvenience that the cost is increased by the provision of such an additional heater. Further, heating rollers of some sizes do not allow three or more heaters to be accommodated within each of them for the reason of space.